雷射投影系統中光斑現象的研究

Abstract

由於以雷射當作光源的光學成像系統中會有一個嚴重影響成像品質的“光斑 (speckle)”問題，減弱或消除光斑的影響便成了此一領域的共同目標之一，在本論文中我們以液晶空間光調變器 (spatial light modulator, SLM) 為主設計一個模擬雷射投影的雛型系統，除了大幅簡化系統架構，並可抑制光斑影響，提高影像品質，我們並以此系統探討降低光斑的方法及將其量測出來。 本論文中我們將介紹投影顯示的概述與投影系統的技術，之後介紹光斑的現象與原理，其中我們以光斑對比 (speckle contrast, SC) 來作為我們判別光斑嚴重程度的重要參數，除此之外我們也介紹一般常用來消除光斑的方法，即以移動的擴散片 (diffuser) 放置在系統中，或藉由螢幕高頻率的振動，使光強度分佈得到均勻化的效果進而降低光斑對比。 在我們的系統中，以反射式液晶空間光調變器取代一般消除光斑的方法中的擴散片。藉由使用遞迴傅立葉演算法 (iterative Fourier transform algorithm, IFTA) 設計出相位圖，此相位圖可以將雷射光束整形 (beam shaping) 成方形，使其能完整照明在幻燈片上以增進投影效率，同時也利用相位調變的技術來取代轉動的擴散片，如此一來可以避免機械式的轉動或者移動，而且也能以相位調變來降低光斑對比，故在此我們討論以這種方法為降低光斑的主要方法。 在未來的研究上，可以將設計的相位圖優化，讓它在光束整形上可以達到強度更均勻。除此之外也可將投影系統中的幻燈片替換成顯示面板 (display panel) 討論靜態與動態的差異，且討論三種雷射光混色後與單色光時在光斑現象上有何差異，使我們的系統能更接近實際使用的投影機。

In the Master Thesis, we design a laser projection system. There is a serious problem which influence projection image qualities from the illumination source “Laser”, called“Speckle”. Therefore, we study speckle reduction methods and measure it in this thesis. In introduction, we will introduce the projection displays technology and classify projection systems. After that we describe phenomena and principle of speckle. By utilize the “Speckle Contrast (SC) ”to judge how critical the speckle pattern are. Besides, we introduce the speckle reduction methods that commonly used, that is to place a moving diffuser in the system or by vibrating screen with high frequency (let human eyes cannot felt) to suppress SC by averaging the total intensity. After that we consider the laser propagation behavior in our system. And then we use reflective spatial light modulator (SLM) to replace the diffuser which is in the general suppressing speckle method. We design the phase diagrams by (Iterative Fourier Transform Algorithm, IFTA), and they can shape laser from a circle to a square for promoting the projection efficiency by complete illumination in the filmstrip. At the same time, we use the phase modulation technology to replace rotating diffuser. Thus we can avoid mechanical rotation and high-frequency vibration, and have faster modulation speed for suppressing speckle as well. For this reason we discuss this speckle reduction method in this. In future research, we shall optimize these phase diagrams designed for more uniform intensity and more like a square. Besides, we can change filmstrip into display panel to discuss the difference between dynamic and static states, and discuss what difference between three color mixing laser and monochrome laser for making our system closer to the actual projector.